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TEXTBOOK OF BOTANY 



with the northern boundary of the western evergreen forest. Across the 

 plains it is bordered by grasslands and aspen-grass savannas. Eastward 

 it intermingles with the hemlock-hardwood forest of the Great Lakes 

 region and the northern Appalachian Mountains. 



The climate of the region is exceedingly rigorous, but the winter 

 temperature rarely falls to —40° F., and the snowfall is deeper than in 



Fig. 377. A spruce-fir forest in the White Mountain National Forest of New Hamp- 

 shire. Photo from U. S. Forest Service. 



the tundra. In the plains region the soil may freeze to a depth of 10 feet; 

 where the snowfall is greater the depth of freezing is much less. The 

 growing season is a month or two longer than in the tundra. There is a 

 distinct spring and summer period, but the autumn is short and winter 

 follows closely. Frosts may occur during any month of the year. 



The boreal forest has occupied much of its present area only a com- 

 paratively short time, for this is the region which was covered ten to 

 twenty thousand years ago by continental glaciers, similar to the great 



